Gel electrolyte for lithium ion secondary battery, and lithium ion secondary battery

ABSTRACT

An object of the present invention is to provide a gel electrolyte for a lithium ion secondary battery having flame retardancy over a long period and a good capacity maintenance rate. The gel electrolyte for a lithium ion secondary battery according to the exemplary embodiment contains a lithium salt, a copolymer of at least one first monomer selected from compounds represented by chemical formulae (1) and (2) and a second monomer represented by chemical formula (4), at least one oxo-acid ester derivative of phosphorus selected from compounds represented by chemical formulae (5) to (7), and at least one disulfonate ester selected from a cyclic-chain type disulfonate ester represented by chemical formula (8) and a linear-chain type disulfonate ester represented by chemical formula (9).

TECHNICAL FIELD

The present invention relates to a gel electrolyte for a lithium ionsecondary battery, and a battery having the same, and particularly to alithium ion battery concurrently having high safety and good lifecharacteristics.

BACKGROUND ART

Since lithium ion or lithium secondary batteries can achieve high energydensity, these attract attention as power sources for cell phones andnotebook computers, and additionally also as large power sources forelectricity storage and power sources for automobiles.

Although lithium ion or lithium secondary batteries can achieve highenergy density, up-sizing makes the energy density gigantic, and highersafety is demanded. For example, in large power sources for electricitystorage and power sources for automobiles, especially high safety isdemanded. Therefore, there are applied safety measures including thestructural design of cells, packages and the like, protection circuits,electrode materials, additives having an overcharge protection function,the reinforcement of shutdown function of separators, and the like, thussecuring the safety of secondary batteries.

Lithium ion secondary batteries use aprotic solvents such as cycliccarbonates and chain carbonates as an electrolyte solution solvent, andthese carbonates tend to have a low flash point and be combustiblethough having a high dielectric constant and a high ionic conductivityof lithium ions.

A technology is known which uses as an additive a substance which isreductively decomposed at a higher potential than those of carbonatesused as electrolyte solution solvents and forms an SEI (SolidElectrolyte Interface) that is a protection membrane having a highlithium ion permeability. The SEI has large effects on thecharge/discharge efficiency, the cycle characteristics and the safety.The SEI can reduce the irreversible capacity of carbon materials andoxide materials.

As one of means to further enhance the safety of lithium ion secondarybatteries, there is a method in which electrolyte solutions are made tobe flame retardant. Patent Literature 1 discloses an organic electrolytesolution secondary battery which uses a phosphate triester as a mainsolvent of an organic electrolyte solution and in which negativeelectrode contains a carbon material as a main constituting element.Patent Literature 2 discloses that the use of a phosphate triester as anorganic solvent of an electrolyte solution can improve the safety.

Patent Literature 3 discloses a secondary battery in which a nonaqueouselectrolyte solution contains at least one selected from the groupconsisting of phosphate esters, halogen-substituted phosphate esters andcondensed phosphate esters. Patent Literature 4 discloses that the useof a mixed solvent of a specific halogen-substituted phosphate estercompound and a specific ester compound as an electrolyte solutionsolvent can provide an electrolyte solution which has a low viscosityand excellent low-temperature characteristics. Patent Literature 5discloses a production method of a battery which uses a nonaqueouselectrolyte solution added with a vinylene carbonate and 1,3-propanesultone. Patent Literature 6 discloses a battery which has a nonaqueouselectrolyte solution which contains a predetermined amount of phosphateesters having a fluorine atom in the molecular chain, and in which aconcentration of a salt is 1 mol/L or higher, and which viscosity islower than 6.4 mPa·s. It is assumed that making such a constitution canprovide a battery having flame retardancy, self-extinguishability andhigh-rate charge/discharge characteristics.

Patent Literature 7 discloses a nonaqueous electrolyte solution whichcontains at least one phosphate ester derivative represented by apredetermined formula, a nonaqueous solvent and a solute. PatentLiterature 8 discloses that the use of a fluorophosphate ester compoundfor a nonaqueous electrolyte solution can provide an electrolytesolution which is excellent in conductivity and reduction resistance,and which develops high flame retardancy even in a low amount blended.

Patent Literature 9 discloses an electrolyte solution which contains asolvent containing a halogenated ethylene carbonate, a phosphate ester,and at least one phosphorus-containing compound selected from the groupconsisting of phosphate esters and phosphazene compounds. It disclosedthat the use of the electrolyte solution can improve chemical stabilityin high temperatures. Patent Literature 10 discloses a nonaqueouselectrolyte solution in which a lithium salt is dissolved in anonaqueous solvent containing a phosphate ester compound, ahalogen-containing cyclic carbonate ester and a chain carbonate ester.Patent Literature 11 discloses a nonaqueous electrolyte solution whichcontains an organic solvent containing a predetermined amount of afluorine-containing phosphate ester represented by a predeterminedformula, and an electrolyte salt. It disclosed that the electrolytesolution has the noncombustibility and flame retardancy useful for anelectrolyte solution of a lithium secondary battery, has a highsolubility of the electrolyte salt, has a large discharge capacity, andis excellent in charge/discharge cycle characteristics.

Patent Literature 12 describes a composition for a polymer solidelectrolyte containing a fluorine-containing phosphate ester. The PatentLiterature discloses a polymer crosslinking material composed of acombination of an epoxy group- and/or an oxetane ring-containingpolymer, and a cationic polymerization initiator.

CITATION LIST Patent Literature

-   Patent Literature 1: JP2908719B-   Patent Literature 2: JP3425493B-   Patent Literature 3: JP10-255839A-   Patent Literature 4: JP3821495B-   Patent Literature 5: JP2007-059192A-   Patent Literature 6: JP2007-258067A-   Patent Literature 7: JP3422769B-   Patent Literature 8: JP2006-286277A-   Patent Literature 9: JP2007-115583A-   Patent Literature 10: JP3961597B-   Patent Literature 11: JP2008-21560A-   Patent Literature 12: JP2003-238821A

SUMMARY OF INVENTION Technical Problem

However, in Patent Literatures 1 and 2, a phosphate ester is reductivelydecomposed on a carbon negative electrode during a long-period usage,and an increase of resistance occurs due to depositing the reducedsubstance on the electrode, and an increase of resistance occurs due togas generation, and thereby may decrease battery characteristics in somecases. A further problem is that a phosphate ester is reductivelydecomposed during usage, and the suppression effect of the combustion ofthe electrolyte solution is decreased in some cases.

Patent Literatures 3 to 8, though having descriptions of combustibilityof an electrolyte solution and initial characteristics of a battery, donot refer to the long-period reliability of the battery. Further, thereis a problem that also a halogen-substituted phosphate ester and aderivative thereof are reductively decomposed gradually on a negativeelectrode during a long-period usage, and a decrease in batterycharacteristics may occur due to an increase of resistance in somecases, and as a result of the reductive decomposition, the suppressioneffect of the combustion of the electrolyte solution may be decreased insome cases. Particularly even in the case where vinylene carbonate or1,3-propane sultone as an additive is added in order to form an SEI asdescribed in Patent Literature 5, sufficient life may not be provided insome cases. The Literatures do not referred to the suppression effect ofthe combustion over a long period.

Patent Literatures 9 to 11 have a description that a halogen-substitutedcyclic carbonate ester can form a halogen-containing film on a negativeelectrode, and the reductive decomposition of a phosphate ester or ahalogen-substituted phosphate ester can be suppressed. However, if thereductive decomposition of a phosphate ester or a halogen-substitutedphosphate ester is attempted to be suppressed over a long period by ahalogen-substituted cyclic carbonate ester alone, a very large amount ofthe halogen-substituted carbonate ester is needed, thereby may cause adecrease of the ionic conductivity in an electrolyte solution in somecases. Further a large an increase of resistance and a decrease in thecapacity maintenance rate may be caused in a long period in some cases.

Furthermore, since any of Patent Literatures 1 to 11 relate to anelectrolyte solution, the problem of solution leakage is apprehended. Bycontrast, Patent Literature 12 has a description of a polymer solidelectrolyte or a polymer gel electrolyte. However, in a batterydescribed in Patent Literature 12, even if the flame retardancy of anelectrolyte itself is maintained initially, a fluorine-containingphosphate ester is decomposed on a negative electrode as in PatentLiteratures 9 to 11, and the deposition of Li is induced, and thereforethe safety as a cell cannot be maintained in some cases.

Then, an object of an exemplary embodiment is to provide a gelelectrolyte for a lithium ion secondary battery which has flameretardancy over a long period and a good capacity maintenance rate.

Solution to Problem

One of the exemplary embodiments is a gel electrolyte for a lithium ionsecondary battery, comprising:

a lithium salt;

a copolymer of at least one first monomer selected from compoundsrepresented by chemical formulae (1) and (2) and a second monomerrepresented by chemical formula (4);

at least one oxo-acid ester derivative of phosphorus selected fromcompounds represented by chemical formulae (5) to (7); and

at least one disulfonate ester selected from a cyclic-chain typedisulfonate ester represented by chemical formula (8) and a linear-chaintype disulfonate ester represented by chemical formula (9).

In formula (1), R¹ represents H or CH₃; and in formulae (1) and (2), R²represents one of substituents represented by the following formula (3).

In formula (3), R³ represents an alkyl group having 1 to 6 carbon atoms.

In formula (4), R⁴ represents H or CH₃; R⁵ represents —COOCH₃, —COOC₂H₅,—COOC₃H₇, —COOC₄H₉, —COOCH₂CH(CH₃)₂, —COO(CH₂CH₂O)_(n)CH₃,—COO(CH₂CH₂O)_(n)C₄H₉, —COO(CH₂CH₂CH₂O)_(n)CH₃,—COO(CH₂CH(CH₃)O)_(n)CH₃, —COO(CH₂CH(CH3)O)_(n)C₂H₅, —OCOCH₃, —OCOC₂H₅,or —CH₂OC₂H₅; and n represents an integer of 1 to 3.

In formula (5), R¹¹, R¹² and R¹³ each independently represent any groupselected from an alkyl group, an aryl group, an alkenyl group, cyanogroup, phenyl group, an amino group, nitro group, an alkoxy group, acycloalkyl group and a halogen-substituted group thereof. Two of or allof R¹¹, R¹² and R¹³ may be bonded to form a ring structure.

In formula (6), R²¹ and R²² each independently represent any groupselected from an alkyl group, an aryl group, an alkenyl group, cyanogroup, phenyl group, an amino group, nitro group, an alkoxy group, acycloalkyl group and a halogen-substituted group thereof. R²¹ and R²²may be bonded to form a ring structure. X²¹ represents a halogen atom.

In formula (7), R³¹ represents any group selected from an alkyl group,an aryl group, an alkenyl group, cyano group, phenyl group, an aminogroup, nitro group, an alkoxy group, a cycloalkyl group and ahalogen-substituted group thereof. X³¹ and X³² each independentlyrepresent a halogen atom.

In formula (8), Q represents an oxygen atom, a methylene group or asingle bond; A¹ represents a substituted or unsubstituted alkylene groupwhich may be branched and has 1 to 5 carbon atoms, a carbonyl group, asulfinyl group, a substituted or unsubstituted perfluoroalkylene groupwhich may be branched and has 1 to 5 carbon atoms, a substituted orunsubstituted fluoroalkylene group which may be branched and has 2 to 6carbon atoms, a substituted or unsubstituted alkylene group whichcontains an ether bond and may be branched and has 1 to 6 carbon atoms,a substituted or unsubstituted perfluoroalkylene group which contains anether bond and may be branched and has 1 to 6 carbon atoms, or asubstituted or unsubstituted fluoroalkylene group which contains anether bond and may be branched and has 2 to 6 carbon atoms; and A²represents a substituted or unsubstituted alkylene group, a substitutedor unsubstituted fluoroalkylene group or an oxygen atom.

In formula (9), R⁶ and R⁹ each independently represent an atom or agroup selected from hydrogen atom, a substituted or unsubstituted alkylgroup having 1 to 5 carbon atoms, a substituted or unsubstituted alkoxygroup having 1 to 5 carbon atoms, a substituted or unsubstitutedfluoroalkyl group having 1 to 5 carbon atoms, a polyfluoroalkyl grouphaving 1 to 5 carbon atoms, —SO₂X₃ (X3 is a substituted or unsubstitutedalkyl group having 1 to 5 carbon atoms), —SY¹ (Y¹ is a substituted orunsubstituted alkyl group having 1 to 5 carbon atoms), —COZ (Z ishydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms), and a halogen atom; and R⁷ and R⁸ each independentlyrepresent an atom or a group selected from a substituted orunsubstituted alkyl group having 1 to 5 carbon atoms, a substituted orunsubstituted alkoxy group having 1 to 5 carbon atoms, a substituted orunsubstituted phenoxy group, a substituted or unsubstituted fluoroalkylgroup having 1 to 5 carbon atoms, a polyfluoroalkyl group having 1 to 5carbon atoms, a substituted or unsubstituted fluoroalkoxy group having 1to 5 carbon atoms, a polyfluoroalkoxy group having 1 to 5 carbon atoms,hydroxyl group, a halogen atom, —NX⁴X⁵ (X⁴ and X⁵ are each independentlyhydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms), and —NY²CONY³Y⁴ (Y² to Y⁴ are each independentlyhydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms).

One aspect of the exemplary embodiments is the gel electrolyte for alithium ion secondary battery which comprises 5 to 60% by mass of theoxo-acid ester derivative of phosphorus.

One aspect of the exemplary embodiments is the gel electrolyte for alithium ion secondary battery which comprises 0.05 to 10% by mass of thedisulfonate ester.

One aspect of the exemplary embodiments is the gel electrolyte for alithium ion secondary battery which comprises 0.5 to 20% by mass of ahalogen-containing cyclic-chain type carbonate ester.

One aspect of the exemplary embodiments is a lithium ion secondarybattery which comprises the gel electrolyte for a lithium ion secondarybattery.

Advantageous Effects of Invention

An exemplary embodiment can provide a gel electrolyte for a lithium ionsecondary battery which concurrently has high flame retardancy and agood capacity maintenance rate over a long period.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram to illustrate a constitution of a positiveelectrode of a lithium ion secondary battery. FIG. 1( a) is a top planview of the positive electrode, and FIG. 1( b) is a side view of thepositive electrode.

FIG. 2 is a schematic diagram to illustrate a constitution of a negativeelectrode of the lithium ion secondary battery. FIG. 2( a) is a top planview of the negative electrode, and FIG. 2( b) is a side view of thenegative electrode.

FIG. 3 is a diagram to illustrate a constitution of an electrodeassembly after being wound of the lithium ion secondary battery.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an exemplary embodiment will be described in detail.

A gel electrolyte for a lithium ion secondary battery according to theexemplary embodiment comprises a lithium salt, a copolymer as a gelcomponent, an oxo-acid ester of phosphorus, and a disulfonate ester.

The copolymer is formed by polymerizing at least one first monomerselected from compounds having a ring-opening polymerizable functionalgroup and represented by chemical formulae (1) and (2), and a secondmonomer having no ring-opening polymerizable functional group andrepresented by chemical formula (4). The ring-opening polymerizablefunctional group is represented by chemical formula (3).

In formula (1), R¹ represents H or CH₃; and in formulae (1) and (2), R²represents one of substituents represented by the following chemicalformula (3).

In formula (3), R³ represents an alkyl group having 1 to 6 carbon atoms.

In formula (4), R⁴ represents H or CH₃; R⁵ represents —COOCH₃, —COOC₂H₅,—COOC₃H₇, —COOC₄H₉, —COOCH₂CH(CH₃)₂, —COO(CH₂CH₂O)_(n)CH₃,—COO(CH₂CH₂O)_(n)C₄H₉, —COO(CH₂CH₂CH₂O)_(n)CH₃,—COO(CH₂CH(CH₃)O)_(n)CH₃, —COO(CH₂CH(CH₃)O)_(n)C₂H₅, —OCOCH₃, —OCOC₂H₅,or —CH₂OC₂H₅; and n represents an integer of 1 to 3.

Examples of the first monomer represented by the above formula (1) or(2) include (3-ethyl-3-oxetanyl)methyl methacrylate, glycidylmethacrylate and 3,4-epoxycyclohexylmethyl methacrylate. These may beused singly or concurrently in two or more. Hereinafter, a monomerrepresented by the above formula (1) or (2) may be referred to as amonomer having a ring-opening polymerizable functional group.

Examples of the second monomer represented by the above chemical formula(4) include methyl acrylate, ethyl acrylate, methyl methacrylate, propylmethacrylate, methoxytriethylene glycol methacrylate andmethoxydipropylene glycol acrylate. The monomer represented by the aboveformula (4) may be used singly or concurrently in two or more.Hereinafter, a monomer represented by the above formula (4) may bereferred to as a monomer having no ring-opening polymerizable functionalgroup.

An oxo-acid ester derivative of phosphorus in the exemplary embodimentis at least one compound represented by the following chemical formulae(5) to (7).

In formula (5), R¹¹, R¹² and R¹³ each independently represent any groupselected from an alkyl group, an aryl group, an alkenyl group, cyanogroup, phenyl group, an amino group, nitro group, an alkoxy group, acycloalkyl group and a halogen-substituted group thereof. Two of or allof R¹¹, R¹² and R¹³ may be bonded to form a ring structure.

In formula (6), R²¹ and R²² each independently represent any groupselected from an alkyl group, an aryl group, an alkenyl group, cyanogroup, phenyl group, an amino group, nitro group, an alkoxy group, acycloalkyl group and a halogen-substituted group thereof. R²¹ and R²²may be bonded to form a ring structure. X²¹ represents a halogen atom.

In formula (6), X²¹ is preferably a fluorine atom.

In formula (7), R³¹ represents any group selected from an alkyl group,an aryl group, an alkenyl group, cyano group, phenyl group, an aminogroup, nitro group, an alkoxy group, a cycloalkyl group and ahalogen-substituted group thereof. X³¹ and X³² each independentlyrepresent a halogen atom.

In formula (7), X³¹ and X³² may be identical or different. X³¹ and X³²are preferably fluorine atoms.

The oxo-acid ester derivative of phosphorus according to the exemplaryembodiment may comprise at least one compound represented by one offormulae (5) to (7).

Specific examples of the compound represented by chemical formula (5)are not especially limited to the following, but include phosphateesters such as trimethyl phosphate, triethyl phosphate, tributylphosphate, triphenyl phosphate, dimethyl ethyl phosphate, dimethylpropyl phosphate, dimethyl butyl phosphate, diethyl methyl phosphate,dipropyl methyl phosphate, dibutyl methyl phosphate, methyl ethyl propylphosphate, methyl ethyl butyl phosphate and methyl propyl butylphosphate. A phosphate ester having a halogen-substituted group includestri(trifluoroethyl)phosphate, methyl(ditrifluoroethyl)phosphate,dimethyl(trifluoroethyl)phosphate, ethyl(ditrifluoroethyl)phosphate,diethyl(trifluoroethyl)phosphate, propyl(ditrifluoroethyl)phosphate,dipropyl(trifluoroethyl)phosphate, tri(pentafluoropropyl)phosphate,methyl(dipentafluoropropyl)phosphate,dimethyl(pentafluoropropyl)phosphate,ethyl(dipentafluoropropyl)phosphate,diethyl(pentafluoropropyl)phosphate, butyl(dipentafluoropropyl)phosphateand dibutyl(pentafluoropropyl)phosphate.

Specific examples of the compound represented by chemical formula (6)are not especially limited to the following, but include dimethylfluorophosphonate, diethyl fluorophosphonate, dibutyl fluorophosphonate,diphenyl fluorophosphonate, methyl ethyl fluorophosphonate, methylpropyl fluorophosphonate, methyl butyl fluorophosphonate, ethyl methylfluorophosphonate, propyl methyl fluorophosphonate, butyl methylfluorophosphonate, ethyl propyl fluorophosphonate, ethyl butylfluorophosphonate, propyl butyl fluorophosphonate,di(trifluoroethyl)fluorophosphonate, methyl trifluoroethylfluorophosphonate, ethyl trifluoroethyl fluorophosphonate, propyltrifluoroethyl fluorophosphonate,di(pentafluoropropyl)fluorophosphonate, methyl pentafluoropropylfluorophosphonate, ethyl pentafluoropropyl fluorophosphonate, butylpentafluoropropyl fluorophosphonate, difluorophenyl fluorophosphonateand ethyl fluorophenyl fluorophosphonate.

Specific examples of the compound represented by chemical formula (7)are not especially limited to the following, but include methyldifluorohypophosphite, ethyl difluorohypophosphite, butyldifluorohypophosphite, phenyl difluorohypophosphite, propyldifluorohypophosphite, trifluoroethyl difluorohypophosphite,fluoropropyl difluorohypophosphite and fluorophenyldifluorohypophosphite.

The content of the oxo-acid ester derivative of phosphorus is preferably5 to 60% by mass, and more preferably 10 to 40% by mass, based on thewhole of a gel electrolyte. In the case where the content of theoxo-acid ester derivative of phosphorus is 5% by mass or higher based ona gel electrolyte, the suppression effect of the combustion of theelectrolyte solution can be more effectively exerted. In the case wherethe content is 10% by mass or higher, the suppression effect of thecombustion can become higher. In the case where the content of theoxo-acid ester derivative of phosphorus is 60% by mass or lower, anincrease in resistance is suppressed, thereby improveing batterycharacteristics. Moreover, the effect of reducing reductivedecomposition by the disulfonate ester can be easily realized, and theeffect of suppressing the combustion over a long period can be easilyrealized.

A disulfonate ester in the exemplary embodiment is at least one selectedfrom a cyclic-chain type disulfonate ester represented by chemicalformula (8) and a linear-chain type disulfonate ester represented bychemical formula (9). The disulfonate ester contributes to the formationof an SEI. A disulfonate ester is preferably contained as an additive.

In formula (8), Q represents an oxygen atom, a methylene group or asingle bond; A¹ represents a substituted or unsubstituted alkylene groupwhich may be branched and has 1 to 5 carbon atoms, a carbonyl group, asulfinyl group, a substituted or unsubstituted perfluoroalkylene groupwhich may be branched and has 1 to 5 carbon atoms, a substituted orunsubstituted fluoroalkylene group which may be branched and has 2 to 6carbon atoms, a substituted or unsubstituted alkylene group whichcontains an ether bond and may be branched and has 1 to 6 carbon atoms,a substituted or unsubstituted perfluoroalkylene group which contains anether bond and may be branched and has 1 to 6 carbon atoms, or asubstituted or unsubstituted fluoroalkylene group which contains anether bond and may be branched and has 2 to 6 carbon atoms; and A²represents a substituted or unsubstituted alkylene group, a substitutedor unsubstituted fluoroalkylene group or an oxygen atom.

In formula (9), R⁶ and R⁹ each independently represent an atom or agroup selected from hydrogen atom, a substituted or unsubstituted alkylgroup having 1 to 5 carbon atoms, a substituted or unsubstituted alkoxygroup having 1 to 5 carbon atoms, a substituted or unsubstitutedfluoroalkyl group having 1 to 5 carbon atoms, a polyfluoroalkyl grouphaving 1 to 5 carbon atoms, —SO₂X³ (X3 is a substituted or unsubstitutedalkyl group having 1 to 5 carbon atoms), —SY¹ (Y¹ is a substituted orunsubstituted alkyl group having 1 to 5 carbon atoms), —COZ (Z ishydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms), and a halogen atom; and R⁷ and R⁸ each independentlyrepresent an atom or a group selected from a substituted orunsubstituted alkyl group having 1 to 5 carbon atoms, a substituted orunsubstituted alkoxy group having 1 to 5 carbon atoms, a substituted orunsubstituted phenoxy group, a substituted or unsubstituted fluoroalkylgroup having 1 to 5 carbon atoms, a polyfluoroalkyl group having 1 to 5carbon atoms, a substituted or unsubstituted fluoroalkoxy group having 1to 5 carbon atoms, a polyfluoroalkoxy group having 1 to 5 carbon atoms,hydroxyl group, a halogen atom, —NX⁴X⁵ (X⁴ and X⁵ are each independentlyhydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms), and —NY²CONY³Y⁴ (Y² to Y⁴ are each independentlyhydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms).

Specific examples of a compound represented by chemical formula (8) areshown in Table 1, and specific examples of a compound represented bychemical formula (9) are shown in Table 2, but these compounds are notespecially limited to these examples. These compounds may be used singlyor concurrently in two or more.

TABLE 1

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

TABLE 2

(101)

(102)

(103)

(104)

(105)

(106)

(107)

(108)

(109)

(110)

(111)

(112)

(113)

(114)

(115)

(116)

(117)

(118)

(119)

(120)

A compound represented by chemical formula (8) or chemical formula (9)can be obtained using a production method described in JP05-44946B.

The content of the disulfonate ester is preferably 0.05 to 10% by mass,and more preferably 0.1 to 5% by mass, based on the whole of the gelelectrolyte. In the case where the content of the disulfonate ester is0.05% by mass or higher, an effect of an SEI can be sufficientlyobtained. In the case where the content of the disulfonate ester is 10%by mass or lower, the reductive decomposition of the oxo-acid ester ofphosphorus can be suppressed over a long period, and an increase ofresistance can be suppressed, thereby battery characteristics can beimproved further.

An aprotic solvent may be contained in a gel electrolyte according tothe exemplary embodiment. An aprotic solvent may be contained in anelectrolyte for a lithium ion secondary battery according to theexemplary embodiment. The aprotic solvent includes cyclic-chain typecarbonates such as propylene carbonate (PC), ethylene carbonate (EC),butylene carbonate (BC) and vinylene carbonate (VC), linear-chain typecarbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC),ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphaticcarboxylate esters such as methyl formate, methyl acetate and ethylpropionate, γ-lactones such as γ-butyrolactone, linear-chain type etherssuch as 1,2-ethoxyethane (DEE) and ethoxymethoxyethane (EME),cyclic-chain type ethers such as tetrahydrofuran and2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide,acetamide, dimethylformamide, dioxolane, acetonitrile, propylnitrile,nitromethane, ethylmonoglyme, phosphate triesters, trimethoxymethane,dioxolane derivatives, sulfolane, methylsulfolane,1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylenecarbonate derivatives, tetrahydrofuran derivatives, ethyl ether,anisole, N-methylpyrrolidone, and fluorocarboxylate esters. Theseaprotic organic solvents can be used singly or as a mixture of two ormore, but are not limited thereto.

In a gel electrolyte according to the exemplary embodiment, ahalogen-containing cyclic-chain type carbonate ester may further becontained as an additive. Since the addition of a halogen-containingcyclic-chain type carbonate ester to a gel electrolyte improves theionic conductivity of an electrolyte solution and contributes to theformation of the film, battery characteristics can be maintained and thesuppression effect of the combustion can be obtained over a long period.The halogen-containing cyclic-chain type carbonate ester includes, forexample, a fluorine-containing carbonate. The fluorine-containingcarbonate include linear-chain type one and cyclic-chain type one, andis preferably cyclic-chain type one (hereinafter, abbreviated also tofluorine-containing cyclic-chain type carbonate).

The content of a halogen-containing cyclic-chain type carbonate ester ispreferably in the range of 0.5 to 20% by mass, more preferably 0.1 to10% by mass, still more preferably 0.2 to 8% by mass, and especiallypreferably 1 to 5% by mass, based on the whole of a gel electrolyte.

The fluorine-containing cyclic-chain type carbonate is not especiallylimited, but compounds obtained by fluorinating part of propylenecarbonate, vinylene carbonate and vinylethylene carbonate, and the likemay be used. More specifically, for example, 4-fluoro-1,3-dioxolan-2-one(fluoroethylene carbonate, hereinafter, referred also to as FEC), (cis-or trans-)4,5-difluoro-1,3-dioxolan-2-one,4,4-difluoro-1,3-dioxolan-2-one and 4-fluoro-5-methyl-1,3-dioxolan-2-oneand the like can be used. Above all, fluoroethylene carbonate ispreferable.

An electrolyte contained in a gel electrolyte according to the exemplaryembodiment is not especially limited, but includes, for example, LiPF₆,LiBF₄, LiAsF₆, LiSbF₆, LiCl₄, LiAlCl₄,LiN(C_(n)F_(2n+1)SO₂)(C_(m)F_(2m+1)SO₂) (n and m are natural numbers),and LiCF₃SO₃.

A gel electrolyte according to the exemplary embodiment can be obtained,for example, by the following Step A and Step B. Step A: a step ofsynthesizing a copolymer of a first monomer represented by formula (1)or formula (2) and a second monomer represented by formula (4). Step B:a step of gelating a pregel solution comprising a lithium salt, thecopolymer obtained in Step A, the oxo-acid ester derivative ofphosphorus, the disulfonate ester, and the aprotic solvent in thepresence of a cationic polymerization initiator.

In the above Step A, the copolymer can be synthesized using a radicalpolymerization initiator. The radical polymerization initiator includesazo-based initiators such as N,N-azobisisobutyronitrile and dimethylN,N′-azobis(2-methylpropionate), and organic peroxide-based initiatorssuch as benzoyl peroxide and lauroyl peroxide. Since such a radicalpolymerization initiator bonds to terminals of the copolymer of thefirst monomer represented by the above formula (1) or (2) and the secondmonomer represented by the above formula (4) along with the initiationof the reaction, and is thereby inactivated, the radical polymerizationinitiator never again causes any reaction by reheating after thecompletion of the reaction.

In the above Step B, the cationic polymerization initiator is notespecially limited, but includes, for example, various types of oniumsalts (for example, cationic salts such as of ammonium and phosphonium,and anionic salts such as of —BF₄, —PF₆ and —CF₃SO₃), and lithium saltssuch as LiBF₄ and LiPF₆.

For a gel electrolyte according to the exemplary embodiment, no radicalpolymerization initiator such as an organic peroxide is needed in Step Bof obtaining the gel electrolyte. Therefore, the oxo-acid esterderivative of phosphorus and the disulfonate ester are not decomposed inthe gelation step. Additionally since the radical polymerizationinitiator is unnecessary for a battery, battery characteristics does notfall due to the influence of leftovers after the polymerization. Since agel electrolyte poses no apprehension of solution leakage as compared toan electrolyte solution, and gives good close adhesivity between bothelectrodes of a positive electrode and a negative electrode and aseparator, life characteristics good over a long period can be provided.

A gel electrolyte according to the exemplary embodiment can decrease theamount of gases generated at first-time charging, which is preferablealso from the viewpoint of the safety. The reason of the decrease of theamount of gases generated is presumably because the concurrent presenceof an oxo-acid ester derivative of phosphorus and a disulfonate ester inthe gel electrolyte can form an SEI incorporating part of the oxo-acidester derivative of phosphorus by a reaction mechanism which isdifferent from an SEI formation by a gel electrolyte containing only adisulfonate ester. It is also presumed that since the reduction of theoxo-acid ester derivative of phosphorus present in the gel electrolytecan be suppressed on the SEI thus formed, the SEI by the disulfonateester incorporating the oxo-acid ester derivative of phosphorus isfirmly formed and the suppression effect of the reductive decompositionon some components including the oxo-acid ester derivative of phosphorusin the gel electrolyte probably becomes large. The effect presumablygives good life characteristics. High safety can further be providedover a long period because an oxo-acid ester derivative of phosphoruscan be suppressed in reductive decomposition over a long period.

As a negative electrode active substance contained in a negativeelectrode of a lithium ion secondary battery having the gel electrolyteaccording to the exemplary embodiment, for example, one or two or moresubstances can be used which are selected from the group consisting ofmetallic lithium, lithium alloys and materials capable of occluding andreleasing lithium. As the material capable of occluding and releasinglithium ions, a carbon material or an oxide can be used.

As the carbon materials, graphite, amorphous carbon, diamond-likecarbon, carbon nanotubes and the like which occlude lithium, andcomposite materials thereof can be used. Particularly graphite ispreferable because graphite has high electron conductivity, and isexcellent in the adhesivity with a current collector composed of a metalsuch as copper, and in the voltage flatness, and contains only a lowcontent of impurities because of being formed at a high treatmenttemperature, which are advantageous for improvement of the negativeelectrode performance. A composite material of a high-crystallinegraphite and a low-crystalline amorphous carbon, and the like canfurther be used.

As the oxide, one of silicon oxide, tin oxide, indium oxide, zinc oxide,lithium oxide, phosphoric acid and boric acid or a composite thereof canbe used. Particularly silicon oxide can be preferably used. Thestructure preferably has an amorphous state. This is because siliconoxide is stable and causes no reaction with other compounds, and becausethe amorphous structure does not induces deterioration caused bynon-uniformity such as crystal grain boundaries and defects. Asfilm-formation method, vapor-deposition method, a CVD method and asputtering method and the like can be used.

The lithium alloy is constituted of lithium and a metal which can bealloyed with lithium. The lithium alloy is constituted of, for example,a binary, ternary, or more multi-metal alloy of metals such as Al, Si,Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn and La, with lithium.Particularly, metallic lithium and lithium alloy preferably have anamorphous state. This is because the amorphous structure hardly causesdeterioration caused by non-uniformity such as crystal grain boundariesand defects.

Metallic lithium and lithium alloy can be suitably formed by a systemincluding a melt cooling system, a liquid quenching system, an atomizingsystem, a vacuum vapor-deposition system, a sputtering system, a plasmaCVD system, an optical CVD system, a thermal CVD system and a sol-gelsystem.

A positive electrode active substance contained in a positive electrodeof a lithium ion secondary battery having the gel electrolyte for alithium ion secondary battery according to the exemplary embodimentincludes, for example, lithium-containing composite oxides such asLiCoO₂, LiNiO₂ and LiMn₂O₄. As the positive electrode active substance,material which is obtained by replacing a part of transition metal ofthe lithium-containing composite oxides with other element can be used.

A lithium-containing composite oxide having a plateau of 4.5 V or highervs. a counter electrode potential of metallic lithium may be used.Examples of the lithium-containing composite oxide include a spinel-typelithium-manganese composite oxide, an olivine-type lithium-containingcomposite oxide and an inverse-spinel-type lithium-containing compositeoxide. The lithium-containing composite oxide includes, for example, acompound represented by Li_(a)(M_(x)Mn_(2-x))O₄ (here, 0<x<2; 0 <a<1.2;and M is at least one selected from the group consisting of Ni, Co, Fe,Cr and Cu).

In a battery constitution of a lithium ion secondary battery accordingto the exemplary embodiment, as an electrode structure, a laminated bodyor a wound body can be utilized, and as an outer package, an aluminumlaminate outer package or a metal outer package can be used. Further,the battery capacity is not limited.

Since the gel electrolyte according to the exemplary embodiment does notneed a radical polymerization initiator such as an organic peroxide forgelation, an oxo-acid ester derivative of phosphorus and a disulfonateester are not decomposed during thermal polymerization. Further sincethe radical polymerization initiator is unnecessary for a battery,battery characteristic does not fall due to the influence of leftoversafter the polymerization. The solution leakage can be prevented becauseof a gel electrolyte.

According to the exemplary embodiment, a disulfonate ester which formsan SEI having a very large suppression effect of the reductivedecomposition is contained, thereby can suppress the reductivedecomposition of an oxo-acid ester derivative of phosphorus on anegative electrode active substance. The concurrent presence of thedisulfonate ester can suppress an increase of resistance due to thereductive decomposition of the oxo-acid ester derivative of phosphorusover a long period, and thereby can provide good life characteristicsover a long period.

According to the exemplary embodiment, since the reductive decompositionof an oxo-acid ester derivative of phosphorus can be suppressed over along period, the oxo-acid ester derivative of phosphorus can be presentin an effective amount to suppress the combustibility in a gelelectrolyte even after a long period usage; therefore, high safety canbe provided over a long period.

Additionally, since the gel electrolyte has no apprehension of solutionleakage, and close adhesivity between both electrodes of a negativeelectrode and a positive electrode and a separator is good, good lifecharacteristics can be provided over a long period.

Further in the exemplary embodiment, the amount of generated gas in thefirst-time charging tends to decrease. The reason of the decrease of theamount of generated gas is presumably because, in the exemplaryembodiment, the concurrent presence of an oxo-acid ester derivative ofphosphorus and a disulfonate ester in the gel electrolyte can form anSEI incorporating part of the oxo-acid ester derivative of phosphorus bya reaction mechanism which is different from an SEI formation in anonaqueous electrolyte solution containing only a disulfonate ester. Itis also presumed that since further reductive decomposition of theoxo-acid ester derivative of phosphorus present in the gel electrolytecan be suppressed on the SEI thus formed, the SEI by the disulfonateester incorporating the oxo-acid ester derivative of phosphorus isfirmly formed and the suppression effect of the reductive decompositionon gel electrolyte components including the oxo-acid ester derivative ofphosphorus probably becomes large. The effect presumably gives good lifecharacteristics. High safety can further be provided over a long periodbecause an oxo-acid ester derivative of phosphorus can be suppressed inreductive decomposition over a long period.

EXAMPLES

Hereinafter, the exemplary embodiments will be described in detail byway of Examples by reference to drawings. The present invention is notlimited to the following Examples.

FIG. 1 is a schematic diagram illustrating a constitution of a positiveelectrode of a lithium ion battery of Example 1. FIG. 2 is a schematicdiagram illustrating a constitution of a negative electrode of thelithium ion battery of Example 1. FIG. 3 is a schematic cross-sectionaldiagram illustrating a constitution of an electrode assembly after beingwound of the lithium ion battery of Example 1.

Example 1

First, fabrication of a positive electrode will be described withreference to FIG. 1. 85% by mass of LiMn₂O₄ as a positive electrodeactive substance, 7% by mass of acetylene black as a conductiveauxiliary material and 8% by mass of polyvinylidene fluoride as a binderwere mixed to obtain a mixture, and then N-methylpyrrolidone was addedto the mixture, and further mixed to thereby prepare a positiveelectrode slurry. The positive electrode slurry was applied on bothsurfaces of an Al foil 2 having a thickness of 20 μm as a currentcollector by a doctor blade method so that the thickness after rollpressing became 160 μm, dried at 120° C. for 5 min, and thereaftersubjected to a roll pressing step to thereby form positive electrodeactive substance-applied portions 3. Positive electrode activesubstance-unapplied portions 5, on which no positive electrode activesubstance was applied, were provided on either surface of both endportions of the foil. A positive electrode conductive tab 6 was providedon one of the positive electrode active substance-unapplied portions 5.A positive electrode active substance-one surface-applied portion 4,which was a part of one surface having the positive electrode activesubstance applied only on the one surface, was provided adjacent to thepositive electrode active substance-unapplied portion 5 provided withthe positive electrode conductive tab 6. A positive electrode 1 wasfabricated by the above method.

Fabrication of a negative electrode will be described with reference toFIG. 2. 90% by mass of graphite as a negative electrode activesubstance, 1% by mass of acetylene black as a conductive auxiliarymaterial and 9% by mass of polyvinylidene fluoride as a binder weremixed to obtain a mixture, and N-methylpyrrolidone was added to themixture, and further mixed to thereby prepare a negative electrodeslurry. The negative electrode slurry was applied on both surfaces of aCu foil 8 having a thickness of 10 μm to become a current collector by adoctor blade method so that the thickness after roll pressing became 120μm, dried at 120° C. for 5 min, and thereafter subjected to a rollpressing step to thereby form negative electrode activesubstance-applied portions 9. Negative electrode activesubstance-unapplied portions 11, on which no negative electrode activesubstance was applied, were provided on either surface of both endportions of the foil. A negative electrode conductive tab 12 wasprovided on one of the negative electrode active substance-unappliedportions 11. A negative electrode active substance-one surface-appliedportion 10, which was a part of one surface having the negativeelectrode active substance applied only on the one surface, was providedadjacent to the negative electrode active substance-unapplied portion 11provided with the negative electrode conductive tab 12. A negativeelectrode 7 was fabricated by the above method.

Fabrication of an electrode assembly will be described with reference toFIG. 3. A fused and cut portion of two sheets of separator 13 composedof a polypropylene microporous membrane having a membrane thickness of25 μm and a porosity of 55% subjected to a hydrophilicizing treatmentwas fixed and wound to a winding core of a winding apparatus, and frontends of the positive electrode 1 (FIG. 1) and the negative electrode 7(FIG. 2) were introduced. The side of the positive electrode opposite tothe connection portion of the positive electrode conductive tab 6 wasmade to be front end side of the positive electrode 1 and the side ofthe connection portion of the negative electrode conductive tab 12 weremade to be the front end side of the negative electrode 7. The negativeelectrode was disposed between the two sheets of the separator, and thepositive electrode was disposed on the upper surface of the separator,and these were wound by rotating the winding core to thereby form anelectrode assembly (hereinafter, referred to as a jelly roll (J/R)).

The J/R was accommodated in an embossed laminate outer package, thepositive electrode conductive tab 6 and the negative electrodeconductive tab 12 were led out, and one side of the laminate outerpackage was folded back, and thermally fused with a portion for solutioninjection being left unfused.

Preparation of a polymer solution will be described. Ethyl acrylate as asecond monomer and (3-ethyl-3-oxetanyl)methyl methacrylate as a firstmonomer were added in respective proportions of 74% by mass and 26% bymass. A reaction solvent of ethylene carbonate (EC)/diethyl carbonate(DEC)=30/70 (in volume ratio) was used. 2,500 ppm ofN,N′-azobisisobutyronitrile as a polymerization initiator with respectto the monomer weight was added. The mixture was heated and reacted at65 to 70° C. under introduction of a dry nitrogen gas, and thereaftercooled to room temperature. Thereafter, EC/DEC=30/70 (in volume ratio)as a diluent solvent was added, and the whole was stirred and dissolveduntil the whole became homogeneous, to thereby obtain a polymer solutionwhich contains 4.0% by mass of polymer having molecular weight of200,000 and EC/DEC=30/70 (in volume ratio).

Preparation of a pregel solution will be described. A pregel solutionwas prepared by using the polymer solution containing EC:DEC=30/70 (involume ratio) with the 4.0% by mass of polymer having molecular weightof 200,000, tri(2,2,2-trifluoroethyl) phosphate, (EC)/(DEC)=30/70 (involume ratio), a compound No. 2 in Table 1, and LiPF₆. Morespecifically, these materials were blended so that the polymer was 2.0%by mass; tri(2,2,2-trifluoroethyl)phosphate was 5.0% by mass; thecompound No. 2 in Table 1 was 2.0% by mass; and LiPF₆ was 1.2 mol/L, tothereby fabricate the pregel solution.

Then, the pregel solution was injected from the laminate solutioninjection portion, and impregnated under vacuum. Then, the solutioninjection portion was thermally fused. Then, the injected pregelsolution was heated and polymerized for gelation at 60° C. for 24 hours.A battery was obtained by the above steps.

A discharge capacity acquired when the obtained battery was CC-CVcharged (an upper-limit voltage: 4.2 V, a current: 0.2 C, a CV time: 1.5hours), and thereafter CC discharged (a lower-limit voltage: 3.0 V, acurrent: 0.2 C) was defined as an initial capacity. The proportion ofthe acquired initial capacity to a design capacity is shown in Table 3.

A cycle test of the obtained battery is carried out by CC-CV charge (anupper-limit voltage: 4.2 V, a current: 1 C, a CV time: 1.5 hours) and CCdischarge (a lower-limit voltage: 3.0 V, a current: 1 C), and either wascarried out at 45° C. The capacity maintenance rate was defined as aproportion of a discharge capacity at the 1,000th cycle to a dischargecapacity at the first cycle. The capacity maintenance rate is shown inTable 3.

A combustion test is carried out by placing the battery after the cycletest at a height of 10 cm above the tip of the flame of a gas burner.The flame retardancy was judged as follows from the state of thevaporizing and burning electrolyte solution solvent. A case where theelectrolyte solution was not ignited: ⊙; a case where even if ignitionoccurred, the fire went out after 2 to 3 sec: ◯; a case where even ifignition occurred, the fire went out within 10 sec: Δ; and a case wherethe fire continued to burn for 10 sec: ×.

Example 2

Example 2 was carried out in the same manner as Example 1, except formixing 10% by mass of tri(2,2,2-trifluoroethyl) phosphate (hereinafter,referred also to as PTTFE) to prepare a pregel solution.

Example 3

Example 3 was carried out in the same manner as Example 1, except formixing 20% by mass of PTTFE to prepare a pregel solution.

Example 4

Example 4 was carried out in the same manner as Example 1, except formixing 40% by mass of PTTFE to prepare a pregel solution.

Example 5)

Example 5 was carried out in the same manner as Example 1, except formixing 20% by mass of di(2,2,2-trifluoroethyl)fluorophosphate(hereinafter, simply referred also to as ditrifluoroethylfluorophosphate) in place of PTTFE to prepare a pregel solution.

Example 6

Example 6 was carried out in the same manner as Example 1, except formixing 20% by mass of 2,2,2-trifluoroethyl difluorophosphate(hereinafter, simply referred also to as trifluoroethyldifluorophosphate) in place of PTTFE to prepare a pregel solution.

Example 7

Example 7 was carried out in the same manner as Example 3, except formixing 2% by mass of a compound No. 4 in place of the compound No. 2 inTable 1 to prepare a pregel solution.

Example 8

Example 8 was carried out in the same manner as Example 3, except formixing 2% by mass of a compound No. 101 in Table 2 in place of thecompound No. 2 in Table 1 to prepare a pregel solution.

Example 9

Example 9 was carried out in the same manner as Example 3, except formixing 2% by mass of a compound No. 102 in Table 2 in place of thecompound No. 2 in Table 1 to prepare a pregel solution.

Example 10

Example 10 was carried out in the same manner as Example 3, except forfurther mixing 2% by mass of fluoroethylene carbonate (FEC) to prepare apregel solution.

Comparative Example 1

Comparative Example 1 was carried out in the same manner as Example 3,except for not mixing the compound No. 2 in Table 1 to prepare a pregelsolution.

Comparative Example 2

Comparative Example 2 was carried out in the same manner as Example 3,except for not mixing the compound No. 2 in Table 1 and mixing 3% bymass of 1,3-propane sultone (PS) to prepare a pregel solution.

Comparative Example 3

Comparative Example 3 was carried out in the same manner as Example 3,except for not mixing the compound No. 2 in Table 1 and mixing 5% bymass of vinylene carbonate (VC) to prepare a pregel solution.

Results of Examples 1 to 10 and Comparative Examples 1 to 3 are shown inTable 3.

TABLE 3 Negative electrode Initial Capacity active Oxo-acid ester ofphosphorus Additive capacity maintenance Flame substance/electrolyteKind Content (%) Kind Content (%) (%) rate (%) retardancy Example 1graphite/gel PTTFE 5 No. 2 2 93 82 ◯ Example 2 graphite/gel PTTFE 10 No.2 2 93 80 ⊙ Example 3 graphite/gel PTTFE 20 No. 2 2 93 80 ⊙ Example 4graphite/gel PTTFE 40 No. 2 2 85 68 ⊙ Example 5 graphite/gelDi(trifluoroethyl) 20 No. 2 2 91 80 ⊙ fluorophosphonate Example 6graphite/gel Trifluoroethyl difluorophosphate 20 No. 2 2 91 79 ⊙ Example7 graphite/gel PTTFE 20 No. 4 2 89 75 ⊙ Example 8 graphite/gel PTTFE 20No. 101 2 90 70 ⊙ Example 9 graphite/gel PTTFE 20 No. 102 2 87 72 ⊙Example 10 graphite/gel PTTFE 20 No. 2 + FEC 4 (2 + 2) 82 85 ⊙Comparative graphite/gel PTTFE 20 — — 93 23 X Example 1 Comparativegraphite/gel PTTFE 20 PS 3 87 49 Δ Example 2 Comparative graphite/gelPTTFE 20 VC 5 83 56 Δ Example 3

In Table 3, No. 2 and No. 4 are compounds No. 2 and No. 4 shown in Table1, respectively; No. 101 and No. 102 are compounds No. 101 and No. 102shown in Table 2, respectively; and FEC is fluoroethylene carbonate, PSis 1,3-propane sultone, and VC is vinylene carbonate.

As shown in Examples 1 to 4, although the capacity maintenance rateafter 1,000th cycle tended to slightly decrease as the content of aphosphate ester was increased, the flame retardancy of the electrolytesolution of the battery after the evaluation was very good. ComparingExamples 3 and 5 to 10 and Comparative Examples 1 to 3 where the sameamount of a phosphate ester is added, in Examples 3 and 5 to 10 where adisulfonate ester is added, good capacity maintenance rates and verygood flame retardancy were obtained. In Example 10 where FEC was furtheradded, a good capacity maintenance rate and very good flame retardancywere obtained.

From the above, since a better SEI could be formed by using adisulfonate ester and an oxo-acid ester derivative of phosphorus, thereductive decomposition of the oxo-acid ester derivative of phosphorusin the gel electrolyte could be suppressed over a long period, therebycan provide good life characteristics, and thereby can provide highflame retardancy over a long period.

Example 11

Example 11 was carried out in the same manner as Example 3, except forusing a silicon-based material in place of the graphite as a negativeelectrode material in Example 3. The fabrication method of the negativeelectrode will be described hereinafter. First, 90% by mass of silicon,1% by mass of acetylene black as a conductive auxiliary agent and 9% bymass of a polyimide binder as a binder were mixed, andN-methylpyrrolidone was added to the mixture and further mixed tothereby prepare a negative electrode slurry. The negative electrodeslurry was applied on both surfaces of a Cu foil having a thickness of10 μm to become a current collector so that the thickness after rollpressing became 80 μm, dried at 120° C. for 5 min, subjected to apressing step, and further additionally dried at 300° C. for 10 min tothereby form negative electrode active substance-applied portions 9.

Example 12

Example 12 was carried out in the same manner as Example 11, except formixing 2% by mass of a compound No. 101 in Table 2 in place of thecompound No. 2 in Table 1 to prepare a pregel solution.

Comparative Example 4

Comparative Example 4 was carried out in the same manner as Example 11,except for not mixing the compound No. 2 in Table 1 and mixing 3% bymass of PS to prepare a pregel solution.

Comparative Example 5

Comparative Example 5 was carried out in the same manner as Example 11,except for not mixing the compound No. 2 in Table 1 and mixing 5% bymass of VC to prepare a pregel solution.

Results of Examples 12 and 13 and Comparative Examples 4 and 5 are shownin Table 4.

TABLE 4 Negative electrode Oxo-acid ester Initial Capacity active ofphosphorus Additive capacity maintenance Flame substance/electrolyteKind Content (%) Kind Content (%) (%) rate (%) retardancy Example 11silicon/gel PTTFE 20 No. 2 2 74 71 ⊙ Example 12 silicon/gel PTTFE 20 No.101 2 65 63 ⊙ Comparative silicon/gel PTTFE 20 PS 3 65 40 Δ Example 4Comparative silicon/gel PTTFE 20 VC 5 56 41 Δ Example 5

In Table 4, No. 2 is the compound No. 2 shown in Table 1; No. 101 is thecompound No. 101 shown in Table 2; and PS is 1,3-propane sultone, and VCis vinylene carbonate.

From Table 4, also in the case where a silicon material was used inplace of a graphite, the SEI by a disulfonate ester could suppressreductive decomposition of an oxo-acid ester derivative of phosphorus tothereby provide a good capacity maintenance rate as lifecharacteristics, and consequently, high flame retardancy could beprovided over a long period.

Example 13

Example 13 was carried out in the same manner as Example 3, except forusing ethyl acrylate as the second monomer and glycidyl methacrylate asthe first monomer.

Example 14

Example 14 was carried out in the same manner as Example 3, except forusing ethyl acrylate as the second monomer and 3,4-epoxycyclohexylmethylmethacrylate as the first monomer.

Example 15

Example 15 was carried out in the same manner as Example 3, except forusing methyl methacrylate as the second monomer and(3-ethyl-3-oxetanyl)methyl methacrylate as the first monomer.

Example 16

Example 16 was carried out in the same manner as Example 3, except forusing methyl methacrylate as the second monomer and glycidylmethacrylate as the first monomer.

Example 17)

Example 17 was carried out in the same manner as Example 3, except forusing methyl methacrylate as the second monomer and3,4-epoxycyclohexylmethyl methacrylate as the first monomer.

Example 18

Example 18 was carried out in the same manner as Example 3, except forusing propyl methacrylate as the second monomer and(3-ethyl-3-oxetanyl)methyl methacrylate as the first monomer.

Example 19

Example 19 was carried out in the same manner as Example 3, except forusing propyl methacrylate as the second monomer and glycidylmethacrylate as the first monomer.

Example 20

Example 20 was carried out in the same manner as Example 3, except forusing propyl methacrylate as the second monomer and3,4-epoxycyclohexylmethyl methacrylate as the first monomer.

Example 21

Example 21 was carried out in the same manner as Example 3, except forusing methoxytriethylene glycol methacrylate as the second monomer and(3-ethyl-3-oxetanyl)methyl methacrylate as the first monomer.

Example 22)

Example 22 was carried out in the same manner as Example 3, except forusing methoxytriethylene glycol methacrylate as the second monomer andglycidyl methacrylate as the first monomer.

Example 23

Example 23 was carried out in the same manner as Example 3, except forusing methoxytriethylene glycol methacrylate as the second monomer and3,4-epoxycyclohexylmethyl methacrylate as the first monomer.

Results of Example 3 and 13 to 23 are shown in Table 5.

TABLE 5 Oxo-acid ester of phosphorus Additive Initial Capacity ContentContent capacity maintenance Flame Second monomer First monomer Kind (%)Kind (%) (%) rate (%) retardancy Example 3 ethyl acrylate(3-ethyl-3-oxetanyl)methyl PTTFE 20 No. 2 2 93 80 ⊙ methacrylate Example13 ethyl acrylate glycidyl methacrylate PTTFE 20 No. 2 2 92 77 ⊙ Example14 ethyl acrylate 3,4-epoxycyclohexylmethyl PTTFE 20 No. 2 2 92 78 ⊙methacrylate Example 15 methyl methacrylate (3-ethyl-3-oxetanyl)methylPTTFE 20 No. 2 2 93 77 ⊙ methacrylate Example 16 methyl methacrylateglycidyl methacrylate PTTFE 20 No. 2 2 93 76 ⊙ Example 17 methylmethacrylate 3,4-epoxycyclohexylmethyl PTTFE 20 No. 2 2 92 76 ⊙methacrylate Example 18 propyl methacrylate (3-ethyl-3-oxetanyl)methylPTTFE 20 No. 2 2 92 78 ⊙ methacrylate Example 19 propyl methacrylateglycidyl methacrylate PTTFE 20 No. 2 2 92 78 ⊙ Example 20 propylmethacrylate 3,4-epoxycyclohexylmethyl PTTFE 20 No. 2 2 93 77 ⊙methacrylate Example 21 methoxytriethylene (3-ethyl-3-oxetanyl)methylPTTFE 20 No. 2 2 93 76 ⊙ glycol methacrylate methacrylate Example 22methoxytriethylene glycidyl methacrylate PTTFE 20 No. 2 2 92 77 ⊙ glycolmethacrylate Example 23 methoxytriethylene 3,4-epoxycyclohexylmethylPTTFE 20 No. 2 2 91 76 ⊙ glycol methacrylate methacrylate

In Table 5, No. 2 is the compound No. 2 in Table 1.

From the above, the SEI by a disulfonate ester, not depending on thepolymer constitution, could suppress the reductive decomposition of anoxo-acid ester of phosphorus over a long period to be thereby able toprovide good life characteristics, and consequently, high safety couldbe provided over a long period.

Comparative Example 6

Comparative Example 6 was carried out in the same manner as Example 1,except for fabricating the gel electrolyte of Example 1 as follows.

First, a pregel solution was fabricated by mixing 1.2 mol/L of LiPF₆,EC/DEC=30/70 (in volume ratio), 5% by mass of PTTFE, 2% by mass of thecompound No. 2 in Table 1, 3.8% by mass and 1% by mass of triethyleneglycol diacrylate and trimethylolpropane triacrylate as gelating agents,respectively, and 0.5% by mass of t-butyl peroxypivalate as apolymerization initiator. First, the PTTFE and the compound No. 2 inTable 1 were mixed in a solution containing LiPF₆ and the EC/DEC=30/70(in volume ratio), and then the gelating agent was added and well mixed,and thereafter the polymerization initiator was mixed.

Then, the pregel solution was injected from the solution injectionportion, and impregnated under vacuum. Then, the pregel solution waspolymerized at 80° C. for 2 hours for gelation. A battery was obtainedby the above steps, and the measurements were carried out in the samemanner as Example 1.

Comparative Example 7

Comparative Example 7 was carried out in the same manner as ComparativeExample 6, except for mixing 10% by mass of PTTFE.

Comparative Example 8

Comparative Example 8 was carried out in the same manner as ComparativeExample 6, except for mixing 20% by mass of PTTFE.

Results of Examples 1 to 3 and Comparative Examples 6 to 8 are shown inTable 6.

TABLE 6 Oxo-acid ester of phosphorus Additive Initial CapacityPolymerization initiator for Content Content capacity maintenance FlameMonomer gelation Kind (%) Kind (%) (%) rate (%) retardancy Example 1ethyl acrylate, cationic polymerization initiator PTTFE 5 No. 2 2 93 82◯ Example 2 (3-ethyl-3-oxetanyl)methyl (LiPF₆) 10 No. 2 2 93 80 ⊙Example 3 methacrylate 20 No. 2 2 93 80 ⊙ Comparative triethylene glycolt-butyl peroxypivalate PTTFE 5 No. 2 2 76 69 X Example 6 diacrylate,Comparative trimethylolpropane 10 No. 2 2 75 55 Δ Example 7 triacrylateComparative 20 No. 2 2 71 51 ◯ Example 8

In Table 6, No. 2 is the compound No. 2 in Table 1.

It is found that Comparative Examples 6 to 8 exhibited a lowersuppression effect of combustion after the cycle than Examples 1 to 3even with the same amount of a phosphate ester added. It is presumedthat in Comparative Examples 6 to 8, the flame retardancy was decreaseddue to the decomposition of a phosphate ester by a polymerizationinitiator in the gel electrolyte. It is also presumed that since an SEIis difficult to be normally formed due to the decomposition of aphosphate ester by a polymerization initiator, the cycle maintenancerate decreased. By contrast, the battery comprising the gel electrolyteaccording to the exemplary embodiment could suppress the reductivedecomposition of an oxo-acid ester of phosphorus over a long period tothereby provide good life characteristics, and consequently, high safetycould be provided over a long period.

The present application claims the priority to Japanese PatentApplication 2010-290545, filed on Dec. 27, 2010, the disclosure of whichis herein incorporated by reference in its entirety.

Hitherto, the present invention has been described by reference toexemplary embodiments and Examples, but the present invention is notlimited to the exemplary embodiments and the Examples. Variousmodifications and changes understandable by those skilled in the art maybe made in the constitution and detail of the present invention withinthe scope of the present invention.

INDUSTRIAL APPLICABILITY

The exemplary embodiments can be utilized, additionally, for energystorage devices such as electric double-layer capacitors and lithium ioncapacitors.

REFERENCE SIGNS LIST

-   1: positive electrode-   2: Al foil-   3: positive electrode active substance applied portion-   4: positive electrode active substance-one surface-applied portion-   5: positive electrode active substance-unapplied portion-   6: positive electrode conductive tab-   7: negative electrode-   8: Cu foil-   9: negative electrode active substance applied portion-   10: negative electrode active substance-one surface-applied portion-   11: negative electrode active substance-unapplied portion-   12: negative electrode conductive tab-   13: insulating porous sheet-   14: positive electrode active substance layer-   15: negative electrode active substance layer

1. A gel electrolyte for a lithium ion secondary battery, comprising: alithium salt; a copolymer of at least one first monomer selected fromcompounds represented by chemical formulae (1) and (2) and a secondmonomer represented by chemical formula (4); at least one oxo-acid esterderivative of phosphorus selected from compounds represented by chemicalformulae (5) to (7); and at least one disulfonate ester selected from acyclic-chain-type disulfonate ester represented by chemical formula (8)and a linear-chain-type disulfonate ester represented by chemicalformula (9):

wherein in formula (1), R¹ represents H or CH₃; and in formulae (1) and(2), R² represents one of substituents represented by the followingformula (3),

wherein in formula (3), R³ represents an alkyl group having 1 to 6carbon atoms,

wherein in formula (4), R⁴ represents H or CH₃; R⁵ represents —COOCH₃,—COOC₂H₅, —COOC₃H₇, —COOC₄H₉, —COOCH₂CH(CH₃)₂, —COO(CH₂CH₂O)_(n)CH₃,—COO(CH₂CH₂O)_(n)C₄H₉, —COO(CH₂CH₂CH₂O)_(n)CH₃,—COO(CH₂CH(CH₃)O)_(n)CH₃, —COO(CH₂CH(CH₃)O)_(n)C₂H₅, —OCOCH₃, —OCOC₂H₅,or —CH₂OC₂H₅; and n represents an integer of 1 to 3,

wherein in formula (5), R¹¹, R¹² and R¹³ each independently representany group selected from an alkyl group, an aryl group, an alkenyl group,cyano group, phenyl group, an amino group, nitro group, an alkoxy group,a cycloalkyl group and a halogen-substituted group thereof; and two ofor all of R¹¹, R¹² and R¹³ may be bonded to form a ring structure,

wherein in formula (6), R²¹ and R²² each independently represent anygroup selected from an alkyl group, an aryl group, an alkenyl group,cyano group, phenyl group, an amino group, nitro group, an alkoxy group,a cycloalkyl group and a halogen-substituted group thereof; R²¹ and R²²may be bonded to form a ring structure; and X²¹ represents a halogenatom,

wherein in formula (7), R³¹ represents any group selected from an alkylgroup, an aryl group, an alkenyl group, cyano group, phenyl group, anamino group, nitro group, an alkoxy group, a cycloalkyl group and ahalogen-substituted group thereof; and X³¹ and X³² each independentlyrepresent a halogen atom,

wherein in formula (8), Q represents an oxygen atom, a methylene groupor a single bond; A¹ represents a substituted or unsubstituted alkylenegroup which may be branched and has 1 to 5 carbon atoms, a carbonylgroup, a sulfinyl group, a substituted or unsubstitutedperfluoroalkylene group which may be branched and has 1 to 5 carbonatoms, a substituted or unsubstituted fluoroalkylene group which may bebranched and has 2 to 6 carbon atoms, a substituted or unsubstitutedalkylene group which contains an ether bond and may be branched and has1 to 6 carbon atoms, a substituted or unsubstituted perfluoroalkylenegroup which contains an ether bond and may be branched and has 1 to 6carbon atoms, or a substituted or unsubstituted fluoroalkylene groupwhich contains an ether bond and may be branched and has 2 to 6 carbonatoms; and A² represents a substituted or unsubstituted alkylene group,a substituted or unsubstituted fluoroalkylene group or an oxygen atom,and

wherein in formula (9), R⁶ and R⁹ each independently represent an atomor a group selected from hydrogen atom, a substituted or unsubstitutedalkyl group having 1 to 5 carbon atoms, a substituted or unsubstitutedalkoxy group having 1 to 5 carbon atoms, a substituted or unsubstitutedfluoroalkyl group having 1 to 5 carbon atoms, a polyfluoroalkyl grouphaving 1 to 5 carbon atoms, —SO₂X³ (X³ is a substituted or unsubstitutedalkyl group having 1 to 5 carbon atoms), —SY¹ (Y¹ is a substituted orunsubstituted alkyl group having 1 to 5 carbon atoms), —COZ (Z ishydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms), and a halogen atom; and R⁷ and R⁸ each independentlyrepresent an atom or a group selected from a substituted orunsubstituted alkyl group having 1 to 5 carbon atoms, a substituted orunsubstituted alkoxy group having 1 to 5 carbon atoms, a substituted orunsubstituted phenoxy group, a substituted or unsubstituted fluoroalkylgroup having 1 to 5 carbon atoms, a polyfluoroalkyl group having 1 to 5carbon atoms, a substituted or unsubstituted fluoroalkoxy group having 1to 5 carbon atoms, a polyfluoroalkoxy group having 1 to 5 carbon atoms,hydroxyl group, a halogen atom, —NX⁴X⁵ (X⁴ and X⁵ are each independentlyhydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms), and —NY²CONY³Y⁴ (Y² to Y⁴ are each independentlyhydrogen atom or a substituted or unsubstituted alkyl group having 1 to5 carbon atoms).
 2. The gel electrolyte for a lithium ion secondarybattery according to claim 1, wherein the gel electrolyte comprises 5 to60% by mass of the oxo-acid ester derivative of phosphorus.
 3. The gelelectrolyte for a lithium ion secondary battery according to claim 1,wherein the gel electrolyte comprises 0.05 to 10% by mass of thedisulfonate ester.
 4. The gel electrolyte for a lithium ion secondarybattery according to claim 1, further comprising 0.5 to 20% by mass of ahalogen-containing cyclic-chain type carbonate ester.
 5. A lithium ionsecondary battery, comprising the gel electrolyte for a lithium ionsecondary battery according to claim
 1. 6. The gel electrolyte for alithium ion secondary battery according to claim 2, wherein the gelelectrolyte comprises 0.05 to 10% by mass of the disulfonate ester. 7.The gel electrolyte for a lithium ion secondary battery according toclaim 2, further comprising 0.5 to 20% by mass of a halogen-containingcyclic-chain type carbonate ester.
 8. The gel electrolyte for a lithiumion secondary battery according to claim 3, further comprising 0.5 to20% by mass of a halogen-containing cyclic-chain type carbonate ester.9. A lithium ion secondary battery, comprising the gel electrolyte for alithium ion secondary battery according to claim
 2. 10. A lithium ionsecondary battery, comprising the gel electrolyte for a lithium ionsecondary battery according to claim
 3. 11. A lithium ion secondarybattery, comprising the gel electrolyte for a lithium ion secondarybattery according to claim 4.